Field of the Invention (Technical Field)
Embodiments of the present invention employ biomimetic multiscale self assembly and materials such as membranes made therefrom, fabricated using batch and automated manufacturing, in various configurations, to enable aqueous separations and concentration of solutes. Embodiments of the present invention also relate to methods of multiscale self assembly and materials made therefrom where a surfactant mesostructure is preferably simultaneously self assembled and integrated with one or more materials by physical confinement between two or more discrete surfaces and/or by physical confinement on two or more sides.
Description of Related Art
Note that the following discussion may refer to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-à-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Membranes are used to separate ions, molecules, and colloids. For example, ultrafiltration membranes may be used to separate water and molecules from colloids which are 2 k Daltons or larger; ion exchange membranes may be used to separate cations and anions; and thin film composite membranes may be used to separate salt from water. These membranes all use the same separation physics. The permeability of the membrane to a specific class or classes of ions, molecules, colloids, and/or particles is much less than another class or classes of ions, molecules, colloids, and/or particles. For example, ultrafiltration membranes have pores of a specific size which prevents the crossover of molecules and particles of a specific size. This technique is known as size exclusion. Reverse osmosis membranes use solubility differences to separate molecules. In a typical thin film composite membrane, the water is three orders of magnitude more soluble than sodium chloride. The result is a material that has a >100:1 preference of water molecules to salt ions. In practical terms, the material filters water by rejecting 99.7% of sodium chloride.
For most separation membranes the permeability of the membrane is defined as the ratio of solvent flux through the membrane in a given period of time to the area of membrane and the pressure applied to the membrane. Below is the equation governing the flux through a membraneFlux=P*(ΔP−Δπ)where ΔP is the pressure across the membrane, Δπ is the osmotic pressure across the membrane and P is the membrane permeability. The permeability of a membrane is a function of the membrane structure parameter. The structure parameter is
  S  =            τ      ·      t        ɛ  where S is the structure parameter, τ is the tortuosity, t is the thickness, and ϵ is the porosity of the membrane. Turtuosity is defined as the ratio of the distance between two points through the material to the minimum distance between the two points. Since the structure parameter is proportional to the permeability of the membrane, the tortuosity is proportional to the permeability.
Membranes for separations are used in many configurations. For reverse osmosis (RO) and forward osmosis (FO) applications, they are often configured in spiral wound architectures, in which the membrane is wrapped around a hollow core. Water flows from the core into a membrane envelope and then back into the core. For pressure retarded osmosis (PRO), the membrane can also be in a spiral wound configuration. In PRO, water under pressure flows into the membrane envelope, and the osmotic gradient across the membrane pulls more water into the membrane envelope. Membranes for RO, FO, and PRO can also be configured as hollow fibers. In hollow fibers, a hollow porous cylindrical membrane is manufactured. Water flows tangential to the membrane surface and the pores in the fiber enable separation. Membranes can also be manufactured as cartridges typically for the concentration of proteins, viruses, bacteria, sugar, and other biological materials. These membranes can come in cassettes that enable easy concentration of solutes.
For the chloralkali process, batteries and fuel cells, the anode and the cathode are separated by an electrolyte. This electrolyte conducts cations or anions and blocks electrons, liquid anolyte, and/or catholyte. In some devices, the electrolyte is an ion exchange membrane. Typically, an ion exchange membrane will allow for the passage of either cations or anions but not both. Ion exchange membranes can be configured to allow for the passage of either both monovalent and divalent ions or only monovalent ions. Transport across the electrolyte of undesired solutes is known as Membrane Crossover. Membrane Crossover creates overpotential at the anode and/or the cathode, and reduces the current efficiency of the cell. Membrane Crossover is a limiting factor in many devices like direct methanol fuel cells, direct ethanol fuel cells, vanadium redox batteries, iron chrome batteries, flow batteries, etc.
In biology, water drives a class of surfactants called lipids to self assemble in water creating a lipid bilayer which acts as a diffusion barrier into the cell. The permeability of model cellular membranes to water and various low molecular weight solutes has been measured. Typical measurements of the selectivity of a lipid bilayer are performed in aqueous suspensions using osmosis (a.k.a. forward osmosis). Also, the results of these experiments show that a lipid bilayer has greater permeability than commercial osmosis (a.k.a. forward osmosis) membranes. The model cellular membranes are phospholipids self assembled by water into structures called vesicles. A phospholipid has a hydrophilic head group and hydrophobic two fatty acid tails. A vesicle is a spherical, hollow, lipid bilayer between 30 nm and 20,000 nm in diameter. The lipid bilayer creates a physical barrier to the volume of water contained within the vesicle. A typical permeability experiment consists of two steps. The first step is to change the osmotic strength of a solute in the aqueous solution containing the vesicles. The second step is to measure the diffusion of the solute and/or solvent across the lipid bilayer into or out of the vesicles. This experiment is similar to the industrial process of forward osmosis where water is extracted through a membrane using a highly concentrated brine solution.
The results of these experiments show that the hydrophobic core of the bilayer separates various low molecular weight compounds. One mechanism is the sub nanometer porosity created by the interstices between the lipids in the bilayer and the hydrophobic core of the bilayer enable preferential selectively for water, protons, uncharged sub 100 molecular weight organics, and ions in that order. Also, fluctuations in the molecular structure of the bilayer enable faster than expected transport of water and protons. Furthermore, these experiments demonstrated control over selectivity via the chemical structure of the lipids used. Specifically, the separation characteristics of the lipid bilayer are dependent on the length of the lipid's fatty acid tails.